So far all I have been able to find is that this is a problem that other people have experienced in mixed exchange environments. The only reliable fix seems to be removing 2013, which I cannot do yet.

Both exchange servers are in the same domain and in the same site. There exists both a .com and .local suffix in AD for users. The change that caused this issue to come up was moving all mailboxes from the 2013 server to the 2016 server.